It is known that atmospheric air contains compounds which should be removed before the said air is introduced into the heat exchangers of the cold box of an air separation unit, in particular the compounds carbon dioxide (CO.sub.2) and water vapour (H.sub.2 O).
This is because, without such pretreatment of the air to remove its CO.sub.2 and water impurities from it, these impurities are found to condense and solidify to form ice when the air is cooled to cryogenic temperature, which may result in problems of clogging the equipment, in particular the heat exchangers, the distillation columns, etc.
Furthermore, it is also common practice to remove the hydrocarbon impurities which may be present in the air in order to prevent them from being concentrated too much in the bottom of the distillation column or columns, and thus to avoid any risk of explosion.
At present, depending on the case, this pretreatment of the air is carried out by a TSA (Temperature Swing Adsorption) process or by a PSA (Pressure Swing Adsorption) process; the term PSA process is intended to mean PSA processes proper, VSA processes (Vacuum Swing Adsorption), VPSA processes and the like.
Conventionally, a TSA process cycle includes the following steps:
a) purifying the air by adsorbing the impurities at superatmospheric pressure and at ambient temperature, PA1 b) depressurizing the adsorber to atmospheric pressure or below atmospheric pressure, PA1 c) regenerating the adsorbent at atmospheric pressure, in particular using the residual gases or waste gases, typically impure nitrogen which is taken from an air separation unit and is heated to a temperature customarily between 100 and 200.degree. C. using one or more heat exchangers, PA1 d) cooling the adsorbent to ambient or sub-ambient temperature, in particular while continuing to introduce into it the residual gas which is output by the air separation unit but has not been heated, PA1 e) repressurizing the adsorber with purified air which is output, for example, by a different adsorber which is in the production phase. PA1 adsorbents which have a high capacity for adsorbing CO.sub.2, but only in the absence of any water vapour, that is to say adsorbents which are effective at removing CO.sub.2 present in a dry gas, PA1 and adsorbents which are intended specifically to remove water vapour and have only little or no affinity for CO.sub.2. PA1 at least one calcined alumina is obtained by: PA1 a) impregnating an adsorbent based on activated alumina with at least one solution of a salt of at least one alkali or alkaline-earth metal chosen from Li, Na, K, Mg, Ca, Sr and Ba; PA1 b) drying an adsorbent based on impregnated alumina obtained in step a) at a temperature of at least 15.degree. C., preferably at least 80.degree. C.; PA1 c) calcining the adsorbent based on dried alumina obtained in step b) at a temperature of at least 120.degree. C., preferably between 150.degree. C. and 600.degree. C.; PA1 at least one alkali or alkaline-earth metal is selected from calcium, sodium, potassium and mixtures thereof; PA1 the calcined alumina contains from 4 to 10% by weight of alkali or alkaline-earth metal oxide, preferably between 5 and 8% or, depending on the case, at most 5% by weight of alkali or alkaline-earth metal oxide, preferably from 1 to 4%, for example the following metal oxides: K.sub.2 O, Na.sub.2 O and CaO; PA1 the calcined alumina particles have a size of between 1 and 5 mm, preferably between 2 and 4 mm; PA1 the CO.sub.2 and/or water vapour are removed in at least one adsorber and, preferably, in at least two adsorbers operating alternately; PA1 it is selected from the group formed by TSA and PSA processes; PA1 the operation is carried out at an adsorption pressure of from 10.sup.5 to 10.sup.7 Pa, preferably 4.times.10.sup.5 Pa to 5.times.10.sup.6 Pa; PA1 the operation is carried out at a temperature of from 10.degree. C. to 70.degree. C., preferably from 20.degree. C. to 55.degree. C.; PA1 it comprises at least one step of regenerating the adsorbent at a regeneration temperature ranging from 0.degree. C. to 250.degree. C. approximately, preferably between 70.degree. C. and 200.degree. C.; PA1 it comprises at least one step of cryogenically separating at least some of the purified air, preferably a step of cryogenically distilling the purified air.
For its part, a PSA process cycle customarily includes substantially the same steps a), b) and e), but differs from a TSA process by the absence of heating of the residual gas or gases during the regeneration step (step c)), and therefore the absence of step d) and, in general, a shorter cycle time than in a TSA process.
Devices for pretreating air generally comprise two adsorbers, operating alternately, that is to say one of the adsorbers is in the production phase while the other is in the regeneration phase.
Such TSA processes for purifying air are described, in particular, in the documents U.S. Pat. No. 3,738,084 and FR-A-7725845.
In general, the CO.sub.2 and the water vapour are removed on a plurality of beds of adsorbents, namely a first adsorbent intended to preferentially retain water, for example a bed of activated alumina, silica gel or zeolites, and a second bed of adsorbent to preferentially retain CO.sub.2, for example a zeolite. The documents U.S. Pat. No. 5,531,808, U.S. Pat. No. 5,587,003 and U.S. Pat. No. 4,233,038 may in particular be cited.
However, obtaining efficient removal of the CO.sub.2 and water vapour which are contained in air on one and the same adsorbent bed is no easy matter.
By way of explanation, it is known that water has much greater affinity for adsorbents than CO.sub.2.
Therefore, the adsorbent generally retains water more readily than CO.sub.2, and so the larger the amount of water adsorbed, the smaller the amount of CO.sub.2 adsorbed.
In other words, the selectivity of conventional adsorbents is more favourable to water than to CO.sub.2.
Furthermore, to make it possible to regenerate a water-saturated adsorbent, it is common practice to heat this adsorbent to a regeneration temperature in excess of 100.degree. C.
At present, however, very few adsorbents employed on an industrial scale in TSA units have a physico-chemical structure capable of withstanding such hydrothermal treatment for a long period of time; materials of the alumina type belong to this category, while most materials of the zeolite type must be excluded.
In brief, known adsorbents can be classed in two broad categories, namely:
In this regard, the document U.S. Pat. No. 5,232,474 may be cited which describes the use of an activated alumina for drying and decarbonating air using a PSA process, in which it is stipulated that the water is retained by an adsorbent bed height of approximately 175 mm, while to adsorb all the CO.sub.2 it is necessary to provide a bed height of 1020 mm. It will therefore be understood that the CO.sub.2 and the water are not co-adsorbed but are retained by different regions of the adsorbent bed.